Dutch elm disease is a scourge that has destroyed many millions of elm trees in the United States of America, in The Dominion of Canada, in Great Britain, and in the countries of Europe, resulting in economic and aesthetic losses of staggering proportions. The disease made its appearance in The Netherlands in the year 1919, and was first observed in the United States in 1930. Although tremendous efforts have been made, and vast sums of money expended, in attempting to deal with it, no effective answer has yet been found.
As pointed out in an article from The Chicago Tribune, dated July 31, 1983, more than half a million elm trees have died from Dutch elm disease in the Chicago area in a period of twenty-eight years and across the nation the disease has killed 43 million trees since 1930. This article also points out that efforts to keep the trees from becoming infected have been made by annually injecting trees with an antibeetle chemical, but such efforts are doomed unless trees are quickly uprooted and removed once they are infected, the reason being that elms planted close together often share roots and the common root system enables the disease to move from a diseased tree to a healthy one.
An article in the Manchester Guardian Weekly, Manchester, England, for Aug. 14, 1983, under the heading "Dutch Elm Disease `Out of Control`", describes the disease as having killed 20 million trees in Great Britain in the past six years. A British official, when asked what is being done to combat the disease, said "Everybody seems to have given up . . . It is getting worse and there is nothing we can do about it." A Forestry Commission spokesman said:
"The tragedy about the disease is that it no longer exists in many parts of southern England because all the trees that were there have either been felled or been killed."
It is therefore apparent that there is a long felt need for a treatment which will counteract Dutch elm disease and preserve elm trees which are fast disappearing from the face of the earth because of this deadly disease.
The disease is caused by the fungus Ceratocystis ulmi, which lives and reproduces in the living tissues of the tree, at one stage in its life history vast numbers of infectious spores being produced. The spores speedily germinate and, after a brief period of growth, produce another batch of spores. During these activities the fungus secretes a toxin, which affects the cell walls of the conducting tissues of the tree in such a way that the transport of water and of dissolved nutrients is completely inhibited, so that the tree withers and dies.
The progress of the disease is a direct function of the rate at which the fungus spreads through the tissues of the tree, and the external manifestations are quite variable. Sometimes the leaves at the tips of one or more branches begin to wilt and take on a yellow or brown coloration, and then to drop off; sometimes an entire branch dies and may fall off, while the rest of the tree maintains a healthy appearance continuing in this condition for several seasons, though eventually, once the infection has made its entrance it spreads throughout the tree; sometimes an apparently healthy tree suddenly shows signs of the disease and in the course of a couple of weeks is dead--such a happening showing that, in spite of the healthy appearance, it was already heavily infected, probably in its root system. A twig may be removed from a tree, an extract of its sap made, and a culture of that extract cultivated in a petri dish; tests will then show presence or absence of the fungus. If the test is fungus-positive, that shows that the tree is infected, a result that would almost certainly have been already indicated by the visual appearance of that particular twig; if the test should be fungus-negative, that shows only that the twig selected is free of infection, and tells nothing whatever regarding the condition of the tree as a whole, which may even be heavily infected. Similar considerations hold regarding the presence or absence of the brownish coloration of the water-conducting tissues observable in sections of diseased twigs. Furthermore, microscopic examination of twig or branch sections from diseased trees, even up to 2,500 diameters, discloses no observable difference in the conducting tissues from that of healthy trees. What this amounts to is that, at the present time, the only diagnostic symptoms regarding the condition of a tree are its visual appearance; and in the case of a healthy appearance, that may be quite misleading.
Before a tree can become diseased it is necessary that the infecting spores shall be carried to it, and then should make an entrance into its living tissues. Since the spores are not in any way naturally adapted for transportation by air currents, nor do they possess any appendage capable of piercing the layers of outer bark which protect the tree's delicate conducting tissues, the spread of the infection is totally dependent upon some external agency; and this is provided in two forms: (1) root grafting, and (2) activities of beetles of the genus Scolytus.
(1) if a root of an infected tree should make contact with a root of a healthy one, there is always a possibility that a graft will be established. Should this happen, then at once the infecting spores are in contact with the living tissues of the healthy tree, and the way has been opened for its infection.
(2) the Scolytus beetle lives, feeds and breeds in all kinds of elm material, and should that material happen to be infected, then, when the beetle moves to a new location, it carries the infecting spores with it. Since in its feeding habits the beetle carves an opening through the outer bark of an elm twig or branch to reach the sap-carrying conductive tissue, here again the way has been opened up for infection.
The measures which have been taken in the fight against the disease are either designed to protect healthy trees from infection, or else to cure those already infected and restore them to health. A large number of protective measures have been employed, by far the most important and effective one being termed "Sanitation", the aim being to eliminate possible sources of infection. All infected material is removed and destroyed. Twigs and branches showing signs of infection are removed by pruning and then destroyed. An infected tree is promptly cut down, its stump and roots removed, and all the infected material destroyed. The method is costly, and it is radical in that it involves the loss of the tree.
The spread of the disease by root grafting has been countered by attempting to impregnate the ground near the trees with a solution of sodium N-methyl dithio-carbamate. It is virtually impossible to achieve complete impregnation, and rainfall speedily leaches away the fungicide.
Another protective procedure is the injection of fungicides into the living tissues of healthy trees, and a large number of different fungicides have been used, some rare and costly, some injurious and deadly. The one most frequently chosen at the present time is Arbotect-20 S. When used as directed it may provide a measure of protection, though it is not dependable. Since it is phytotoxic it can be used only in very small amounts and at great dilution, one part to forty or fifty parts of water. For an average size tree this means a relatively costly amount of Arbotect-20 S diluted with water to a volume of 3 gallons. Using gravity feed through an eight-orifice system it takes about eleven days to feed this volume of liquid into the tree. For effective protection the injection should be repeated annually.
A different protective approach is to make the attack upon the beetles, the carriers of the infecting spores. Various ways of doing this have been tried. They can be killed by spraying them with methoxy chlor as they are feeding on the elm trees. But however thoroughly the spraying is done some beetles escape; furthermore it is necessary to spray at least twice each season to correspond with the emergence of the two generations of beetles. The program is expensive. A much less expensive method is to trap the beetles by affixing to the elm trees sticky papers baited with a pheromone; but because of the vast numbers of the beetles this has not proved to be very effective. The wasp Dendrosoter, which destroys the Scolytus larvae, has been used as a biological antagonist; the method is cumbersome, expensive and dangerous, and quite ineffective.
When methods of curing the disease are considered it has to be admitted that practically nothing has been accomplished.
Because the disease known as Dutch elm disease is caused by a specific fungus, one who is unfamiliar and has little knowledge concerning the treatment of such disease could argue that a fungicide which is capable of destroying one type of fungus should obviously be capable of destroying a different type of fungus, but such is not the case. Fungicides are necessarily different, depending upon the type of fungus to be destroyed and also the type of tree to which the fungicide is applied. The mere fact that a fungicidal solution might be injected into a tree provides no answer as to the effect it might have on the particular fungus with which said tree is infected, or whether the injection of the fungicide, while killing a particular fungus, might also be phytotoxic and kill the tree itself. There is also a difference between a fungus which is a deadly fungus and actually kills a tree and a fungus which is not a deadly fungus insofar as the tree is concerned, but affects the foliage or other aspects of the growth of the tree as in the case of fruit bearing trees which are afflicted with scab or powdery mildew. For example, Keil, Agrigultural Chemicals, pages 23,24 and 128, April 1965, claims on page 23, 3rd column, first paragraph, particular successes in controlling Xanthomonas pruni by combinations of dimethyl sulfoxide with other chemical compounds, including organic mercuries, quaternary ammonium compounds, dodine, hexachlorophene, Karathane, zinc sulfate, and oxytetracycline (Terramycin), in spray tests on peach trees to control bacterial spot caused by this microorganism. Using a spraying technique, some protection was obtained for periods of 8-10 days (page 24, first column, last paragraph) on healthy greenhouse plants. After the plants have been infected, the treatment was effective only when carried out within 24 hours of the infection (page 24, second column, paragraph 1). Using an injection technique, two or three injections were made at intervals of two to four weeks, and in the most successful cases 50% reduction in scab infection and 20-30% reduction in powdery mildew infection was obtained within ten feet of the injection point (page 24, column 3, paragraph 2) on Rome Beauty apple trees 25 to 30 years old. It is not clear from this article which chemicals were actually injected into the apple trees but it is apparent to one knowledgeable in this art that these treatments were not effective. A 50% reduction in scab infection could hardly be called an effective treatment and a 20-30% reduction of powdery mildew cannot be called an effective treatment. The article also points out "There also was increased injury with most DMSO combinations. Greatest phytotoxicity usually appeared on the limb immediately above and often to the right of the injector. Orchard trees appeared to withstand 1-2% solutions of DMSO alone without injury, but 10% solutions caused slight leaf-tip burn." While the article is too vague with respect to the specific combinations used so that no one could repeat the tests described by the author, it is clear that the results obtained could not be predicted from one type of fungus to another or from one type of tree to another and the author was not attempting to counteract a deadly disease such as Dutch elm disease which invariably kills the tree. Whatever combinations with dimethylsulfoxide that Keil used in his injections (mostly they are unspecified except in the case of Terramycin), they all caused significant injury to the living trees (page 24, third column, second paragraph).
The Keil article abounds with a large number of speculations (page 128, columns 1 and 2) and admits presumptions and speculations will have to be proved as practical under field conditions and none of these presumptions and speculations are concerned with the problem of counteracting Dutch elm disease.
It should be pointed out to those who are unfamiliar with the art that Hill, U.S. Pat. No. 4,335,109, in order to impart increased water repellancy to cut wood in which the tissues are no longer living, has suggested combining water repellant solutions with a fungicide, reciting vast numbers of fungicides including zinc acetate and zinc sulfate, as well as other zinc compounds in the form of aqueous treating solutions so as to inhibit fungicidal deterioration. The treating solutions are simply deposited on the wood and spread with a sponge. The wood is then dried at ambient temperatures and tested for water repellancy. Nothing in this patent has any relevance to the problem of keeping living tissues alive in trees which are subject to Dutch elm disease. Nor is there any suggestion or teaching in this patent that aqueous solutions of zinc salts in combination with dimethyl sulfoxide would be effective in counteracting Dutch elm disease.
In Herschler, U.S. Pat. No. 3,551,554, human tissue is treated with various physiologically active agents in dimethyl sulfoxide and there is a brief mention that dimethylsulfoxide enhances the penetration of plant active agents such as pesticides, dyes, nutrients, hormones and herbicides, but there is no suggestion or teaching in this patent of any kind of a treatment involving fungicides or of any attempt to solve the problem of counteracting Dutch elm disease.
In view of the long felt need for a solution of this problem and the failure of others to solve it, the present invention becomes particularly important.